1. Field of the Invention
The present invention relates to a zoom lens for projection and a projection-type display device on which the zoom lens for projection is mounted.
2. Description of the Related Art
In recent years, projection-type display devices, such as transmission-type or reflection-type liquid crystal display devices and DMD (digital micromirror device) display devices, which use various kinds of light bulbs and have relatively long back focus, were widely used. Further, the performance of such devices has been becoming higher.
The projection-type display device uses, as a projection lens, a zoom lens, which can change the size of an image formed on a screen, in many cases. Recently, there is an increasing demand for a zoom lens that can greatly change the size of the image, in other words, a zoom lens having a higher zoom ratio.
Further, a lens shift function and a widening function of the zoom lens became required. The lens shift function shifts the center of a projected image with respect to the projection-type display device, for example, to the upper side. The widening function can project a large image at a position close to the projection-type display device. Therefore, zoom lenses having wider angles (wider angles of view) are needed.
Further, an optical system or the like using a plurality of light bulbs needs space for inserting a prism that combines, together, light beams of various colors output from the plurality of light bulbs respectively. Therefore, long back focus is required.
Conventionally, this kind of projection-type zoom lens disclosed, for example, in Japanese Unexamined Patent Publication No. 2007-240731 (Patent Document 1) or Japanese Unexamined Patent Publication No. 2007-304268 (Patent Document 2) was known.
However, with respect to the zoom ratio, the angle of view, and the back focus, neither of the zoom lenses disclosed in Patent Document 1 and Patent Document 2 sufficiently satisfies the need in recent year.